Requirements have become quite severe in the design of small high efficiency cooling systems for temperature critical electronic components. In a typical cooling system, air must be moved through a heat sink in a very small package size and with very low generation of noise. Axial fans have been employed in axially adjacent relationship with heat sinks but this is quite inefficient from a space standpoint. With axial fans imbedded in a heat sink performance is still found lacking. Unacceptable levels of noise generation and relatively large power requirements have been encountered.
More recent design approaches integrate the heat sink and drive motor into a single module with a relatively small size that is quite efficient. The cooling system may consist of a heat dissipating base plate directly adjacent to heat generating electronics or a heat pipe, and a multiplicity of small spaced apart heat dissipating elements mounted on the base plate and defining a multiplicity of small air flow passages therebetween. A centrally located cavity in the array of heat dissipating elements receives an electric motor which drives a centrifugal impeller disposed adjacent and about the array of heat dissipating elements. Cooling air is directed downwardly through an opening in an impeller backplate which is carried by the motor and is discharged radially after a right angle turn and passage through the heat dissipating elements.
To date these latter designs have employed centrifugal impellers with “forwardly curved” blades. Such impellers have relatively small blade annulus width to wheel radius ratio and this allows the design to maintain a small overall package diameter. However, the design also has significant disadvantages. First, the flow pattern in a forwardly curved impeller involves the recirculation of air through the blade passages and this is disrupted with an array of small heat dissipating elements placed radially within the impeller and adjacent the blades at their inlet ends. Severe losses in efficiency result. Further, with forwardly curved blades, the air at the discharge end of the blades is accelerated to velocities which are higher than the rotational velocity. This results in the need for a diffuser to convert velocity pressure to static pressure at the impeller discharge. Without such a diffuser or pressure conversion housing, these impellers are quite inefficient and may even be unstable. When a properly designed diffuser is associated with impellers with forwardly curved blades, the result is a package that is usually of excessive size in both radial and axial directions.